1. Field of the Invention
The present invention relates to a phosphor powder, and more particularly, a composite phosphor powder which emits light in a variety of spectrums and produces color temperature, color reproduction index and color precisely tailored to needs.
2. Description of the Related Art
A phosphor is utilized in a wide range of light emitting devices such as a light emitting diode device. The phosphor converts various excitation energies such as photon, electron, heat and electric field into visible light. In order to obtain a desired emission wavelength, an adequate activator ion and a matrix material are synthesized to manufacture a phosphor. An excitation energy transferred through a matrix of the phosphor to the active ion is converted to visible light, thereby emitting light. Here, the emission wavelength is determined by combination of a given matrix and an activator doped therein. As a result, the phosphor carries its own emission wavelength and accordingly is extremely limited in altering the emission wavelength as desired.
FIG. 1 illustrates a spectrum of a conventional red phosphor and FIG. 2 illustrates a spectrum of a conventional green phosphor. FIG. 1 is a graph illustrating an emission spectrum of SrS:Eu phosphor which is a red phosphor. FIG. 2 is a graph illustrating an emission spectrum of SrGa2S4:Eu phosphor which is a green phosphor. As shown in FIGS. 1 and 2, the phosphors each emit light in a specified wavelength range. As just described, only one type of phosphor fails to produce a variety of colors and emission spectrums, and moreover extremely limited types of the phosphors emit light at a desired wavelength.
In a method to overcome this problem, several types of phosphors have been mixed together. For example, in a light emitting device having an LED element (LED chip) mounted thereon, several types of phosphors are mixed to emit light in a broad wavelength range (e.g., white light emission). FIG. 3 illustrates an example thereof.
Referring to FIG. 3, a light emitting device 10 includes a blue LED chip 18 mounted in a recess of a housing 11. Terminal electrodes 12 are connected to the LED chip via bonding wires 19. Over the LED chip 18, a first phosphor (e.g., a red phosphor) 13 and a second phosphor (e.g., a green phosphor) 14 are mixed in a molding resin 15 such as epoxy. However, owing to density differences between the first phosphor 13 and the second phosphor 14, the phosphor of high density (the second phosphor) sinks and the phosphor of low density (the first phosphor) floats (a layering phenomenon). This fails to produce color reproduction index and emission spectrum as originally designed or expected, resulting in uneven color.